Research in inhalation toxicology often requires that test animals be exposed for extended periods of time to carefully controlled test atmospheres which contain known concentrations of test contaminants. A variety of animal exposure chambers are known to exist in the prior art, but they have not proved satisfactory in all respects for applicant's purposes.
Typically, a plurality of animals is placed in a single exposure chamber into which a test atmosphere is discharged. The animals are preferably placed in separate cages which are often stacked vertically. Thus a substantially horizontal catch pan is usually provided beneath each cage to protect the animals in lower cages from the excreta of the animals in the cages thereabove. A persistant problem, however, is that the catch pans can interfere with, and even obstruct, the delivery of the test atmosphere to the test animals where the flow of test atmosphere is from top to bottom. Various methods have been developed to minimize the adverse effects of the catch pans, but no solution has been entirely satisfactory.
One approach to the problem, as disclosed for example in U.S. Pat. Nos. 3,302,615 and 4,201,154, utilizes an inhalation chamber in which a plurality of animal cages are suspended from a mechanism which slowly and continuously moves the cages along a predefined path within the chamber. Such systems, however, are complex and expensive to manufacture and maintain, and are too large and heavy to be easily moved from one location in the lab to another.
Another approach to the problem, disclosed for example in U.S. Pat. No. 4,216,741, utilizes an exposure chamber having a pyramidal or conical top and bottom and in which the test atmosphere is introduced at the apex of the top and is withdrawn adjacent the apex of the bottom, such that the general flow of the test atmosphere through the chamber is vertically downward. The horizontal catch pans are positioned within the chamber so that the barrier effect thereof induces turbulence in the test atmosphere flow which is intended to uniformly distribute the test atmosphere to the test animals. Although the pyramidal or conical surfaces are functionally beneficial, in that they facilitate a uniform and non-turbulent divergence and convergence of the test atmosphere entering and leaving the chamber, the fact that they are separate elements having a unique shape increases the manufacturing cost, complexity and height of the system. Further, the dependence of the design on the barrierlike effect of the catch pans limits the extent to which cages and the associated catch pans can be removed from the system without degrading the system operation.
Another prior art approach is to provide an exposure chamber having opposite side walls which are pyramidal or conical with horizontally outwardly extending apexes and having a test atmosphere inlet at one apex and a test atmosphere outlet at the other apex. The flow of the test atmosphere therethrough is thus generally horizontal, and passes smoothly across the catch pans in a direction parallel to the catch pans. Such systems have also proved to be less than entirely satisfactory, in part because of the effect of gravity on the dust components of certain test atmospheres which limit the extent to which the distribution of the test atmosphere is vertically uniform, and also tends to induce a buildup of dust in the bottom of the chamber. These systems are bulky and require large spaces. An additional disadvantage present in many of the prior art devices, including the system disclosed in U.S. Pat. No. 4,216,741 and discussed above, is that insufficient protection from the contamination is provided for the system operator during transfers of the contaminated animals from the exposure chamber to regular animal housing facilities.
Accordingly, it is an object of this invention to provide an animal inhalation exposure apparatus in which a single exposure chamber houses a number of animals in cages having substantially horizontal excreta trays therebelow and in which a substantially uniform distribution of the test atmosphere to the test animals is effected with a minimal disturbance of the test atmosphere flow.
It is a further object of the invention to provide an animal inhalation exposure apparatus, as aforesaid, wherein the test animals may be transferred between the exposure chamber and a regular animal housing facility by a system operator without exposing the system operator to a contamination hazard.
It is a further object of the invention to provide an animal inhalation exposure apparatus, as aforesaid, which permits generation of the test atmosphere in situ without exposing the system operator to a contamination hazard.
It is a further object of the invention to provide an inhalation exposure apparatus, as aforesaid, in which the exposure chamber has a shape which is simple and inexpensive to construct, but which has divergent and convergent surfaces respectively adjacent the test atmosphere inlet and outlet to facilitate the uniform distribution and flow of the test atmosphere through the exposure chamber.
It is a further object of the invention to provide an inhalation exposure apparatus, as aforesaid, which is mobile and which has a filtering system for removing toxic components from the test atmosphere exhausted from the chamber.